1. Field of the Invention
The present invention relates to a video signal processing apparatus for converting an analog video signal composed of an analog luminance component signal and an analog chrominance component signal into a digital video signal and processing the digital video signal and for converting the processed digital video signal into a digital video signal and outputting the digital video signal.
2. Description of Related Art
In recent years, digital video apparatuses for digitizing analog video signals to record the digital signals on tapes or disks and reproduce the digital signals therefrom have begun to popularly spread. More specifically, digital video cameras and digital video disks are known as typical digital video apparatuses.
These digital video apparatuses cope with not only recording/reproduction of digital video signals but also recording/reproduction of conventional analog video signals.
FIG. 1 is a block diagram showing the arrangement of a conventional video signal processing apparatus for converting an analog video signal into a digital video signal. Referring to FIG. 1, reference numeral 103a denotes a low-pass filter (LPF); 123a, a band-pass filter (BPF); 104a and 125a, clamping circuits; and 105a and 126a, A/D converters.
In the video signal processing apparatus shown in FIG. 1, input signals 101a and 121a are an analog luminance (Y) signal and an analog chrominance (C) signal, respectively. After the luminance signal 101a is gain-controlled by an amplifier 102a to have a predetermined signal level, the luminance signal 103aa is subjected to band limit by the low-pass filter 103a. 
This LPF 103a has an amplitude attenuation characteristic of about xe2x88x9212 dB at a frequency of xc2xd is on the basis of a sampling clock frequency fs of an A/D converter, and also has a group delay time YDLY (nS) in a passing band (see FIG. 3).
A luminance signal 103ba subjected to the predetermined band limit by the LPF 103a is inputted to the clamping circuit 104a. Then, a signal potential of the luminance signal 103ba is set by the clamping circuit 104a to be appropriate to the input range of the A/D converter 105a of the next stage. More specifically, a clamping operation of the clamping circuit 104a is performed such that, even if the average luminance level (APL) of a video signal varies, a pedestal level (black level) of the video signal serving as a reference becomes a fixed DC potential.
The A/D converter 105a converts an analog luminance signal 108a outputted from the clamping circuit 104a into a digital luminance signal 106a at a resolution of, e.g., 8 bits.
Similarly, after the chrominance signal 121a is a gain-controlled by an amplifier 122a to have a predetermined signal level the chrominance signal 123aa is subjected to band limit by the band-pass filter (BPF) 123a. 
A chrominance signal in the NTSC system has fsc=3.58 MHz, and a chrominance signal in the PAL system has fsc=4.43 MHz. The BPF 123a has a group delay time CDLY (nS) in a passing band (see FIG. 4). A chrominance signal 123ba subjected to band limit by the BPF 123a is subjected to delay correction by a delay circuit 124a until the chrominance signal 123ba has the same timing as that of the luminance signal 103ba. 
Thereafter, a signal potential of a chrominance signal 130a outputted from the delay circuit 124a is set by the clamping circuit 125a to be appropriate to the input range of the A/D converter 126a. As in the luminance signal system, an analog chrominance signal 129a is converted into a digital chrominance signal 127a by the A/D converter 126a. 
Here, in a signal processing system for converting an analog video signal into a digital video signal, the characteristics of an analog filter for suppressing aliasing noise generated in sampling by the A/D converter 105a are important factors.
In the conventional signal processing system shown in FIG. 1, when the sampling frequency of the luminance signal 101a is set to be, e.g., 13.5 MHz, in order to keep the quality (frequency characteristic and pulse response characteristic) of a signal component in the passing band good, a group delay characteristic of about 300 to 800 nS is generally set by setting a cutoff frequency or the like.
Since the chrominance signal 121a has a narrow signal band, the BPF 123a is generally constituted by a BPF having the characteristics as shown in FIG. 4. However, since the group delay time CDLY of the BPF 123a is shorter than the group delay time YDLY of the LPF 103a for a luminance signal, the delay circuit 124a must be provided to make the delay time of the chrominance signal 121a equal to the delay time of the luminance signal 101a. 
However, the presence of an analog delay circuit for adjusting signal delay, as in the conventional signal processing system, disadvantageously gives bad influence on suppressing the variations of delay timings of the luminance signal and the chrominance signal and assuring the precision of the delay timings, and degrades the quality of the signals, e.g., an S/N ratio or the like.
In addition, an input/output signal processing system for an analog signal in a digital video apparatus is advantageously constituted by a compact system at high precision and low cost by constituting peripheral circuits by ICs. However, from the viewpoints of the circuit scale and the power consumption, it is disadvantageous to constitute a delay circuit for correcting the timings of a luminance signal and a chrominance signal by an analog circuit.
It is an object of the present invention to provide a video signal processing apparatus capable of solving the above problems.
It is another object of the present invention to provide a video signal processing apparatus capable of reducing the variations of delay timings of a luminance component signal and a chrominance component signal and improving the precision of the delay timings.
To attain the above objects, in accordance with an aspect of the present invention, there is provided a video signal processing apparatus for converting an analog video signal composed of an analog luminance component signal and an analog chrominance component signal into a digital video signal and processing the digital video signal, which comprises (i) first analog/digital conversion means for inputting an analog luminance component signal and converting the inputted analog luminance component signal into a digital luminance component signal and for outputting the digital luminance component signal, (ii) second analog/digital conversion means for inputting an analog chrominance component signal and converting the inputted analog chrominance component signal into a digital chrominance component signal and for outputting the digital chrominance component signal, (iii) luminance component signal frequency band limiting means, disposed on a pre-stage of the first analog/digital conversion means, for limiting a frequency band of the analog luminance component signal to be inputted to the first analog/digital conversion means and for supplying the frequency-band-limited analog luminance component signal to the first analog/digital conversion means, (iv) chrominance component signal frequency band limiting means, disposed on a pre-stage of the second analog/digital conversion means, for limiting a frequency band of the analog chrominance component signal to be inputted to the second analog/digital conversion means and for supplying the frequency-band-limited analog chrominance component signal to the second analog/digital conversion means, the chrominance component signal frequency band limiting means having a group delay characteristic different from a group delay characteristic of the luminance component signal frequency band limiting means by a predetermined delay time, and (v) digital signal processing means for individually processing, as digital signals, the digital luminance component signal outputted from the first analog/digital conversion means and the digital chrominance component signal outputted from the second analog/digital conversion means.
It is still another object of the present invention to provide a video signal processing apparatus capable of assuring the signal quality, such as an S/N ratio, and achieving low cost, space saving, and a reduction in power consumption.
To attain the above object, in accordance with another aspect of the present invention, there is provided a video signal processing apparatus for processing, as a digital signal, a digital video signal composed of a digital luminance component signal and a digital chrominance component signal and for converting the processed digital video signal into an analog video signal and outputting the analog video signal, which comprises (i) digital signal processing means for individually processing, as digital signals, the digital luminance component signal and the digital chrominance component signal and outputting the processed digital luminance component signal and the processed digital chrominance component signal, (ii) first digital/analog conversion means for inputting the digital luminance component signal outputted from the digital signal processing means and converting the inputted digital luminance component signal into an analog luminance component signal and for outputting the analog luminance component signal, (iii) second digital/analog conversion means for inputting the digital chrominance component signal outputted from the digital signal processing means and converting the inputted digital chrominance component signal into an analog chrominance component signal and for outputting the analog chrominance component signal, (iv) luminance component signal frequency band limiting means, disposed on a post-stage of the first digital/analog conversion means, for limiting a frequency band of the analog luminance component signal outputted from the first digital/analog conversion means and for outputting the frequency-band-limited analog luminance component signal, (v) chrominance component signal frequency band limiting means, disposed on a post-stage of the second digital/analog conversion means, for limiting a frequency band of the analog chrominance component signal outputted from the second digital/analog conversion means and for outputting the frequency-band-limited analog chrominance component signal, the chrominance component signal frequency band limiting means having a group delay characteristic different from a group delay characteristic of the luminance component signal frequency band limiting means by a predetermined delay time, and (vi) correction means for correcting a difference in delay time between the analog luminance component signal outputted from the luminance component signal frequency band limiting means and the analog chrominance component signal outputted from the chrominance component signal frequency band limiting means caused by the difference in group delay characteristic between the luminance component signal frequency band limiting means and the chrominance component signal frequency band limiting means.
These and further objects and features of the present invention will be apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.